The present invention relates to a semiconductor module in which bare or packaged semiconductor chips are mounted on a wiring board, a heat spread plate is disposed above the semiconductor chips, and resin is filled around the semiconductor chips so that they are sandwiched between the wiring board and the heat spread plate, as well as a mounting method for the semiconductor module. Particularly, the invention is concerned with a multi-chip module with a plurality of semiconductor chips mounted on a single wiring board, as well as a mounting method for the multi-chip module.
Semiconductor modules are used as high-speed or highly integrated memories in electronic devices such as personal computers, servers, and large-sized computers. These electronic devices now show a tendency toward reduction of size and thickness and are becoming higher in speed and integration degree. With such a tendency, semiconductor modules are also now required to attain high-density mounting, narrow pitch/multi-pin connection, low noise, and low heat resistance.
Under such circumstances, attempts are being made to enhance the mounting density by mounting on a wiring board a plurality of bare chips or chips size (or chip-scale) packages (hereinafter referred to as CSP) each almost equal in size to a chip. Attempts are also being made to attain a low heat resistance by bonding a heat spread plate to the backs of chips after mounting of plural bare chips or CSPs onto a wiring board.
As an example, in U.S. Pat. No. 5,724,729 there is disclosed a multichip module having a structure wherein a plurality of semiconductor chips are mounted on a single wiring board, and a heat spread cap is bonded to the backs of the semiconductor chips with use of a thermally conductive adhesive.
In Japanese Patent Laid-open No. Sho 63-29563 there is disclosed a semiconductor device having a structure wherein a flip chip is face down-bonded to a mullite substrate through bump electrodes, a silicon carbide substrate superior in thermal conductivity is bonded onto the flip chip with use of solder, and an epoxy resin is filled between the mullite substrate and the silicon carbide substrate. There is also described therein a multi-chip module having a structure wherein a plurality of such semiconductor devices as just referred to above are mounted on a single substrate, a heat spread plate is disposed so as to cover the plural semiconductor devices from above, and the heat spread plate and the semiconductor devices are connected together using a comb teeth-like heat transfer part.
According to our study, however, the above conventional techniques involve the following problems.
First, in the multi-chip module described in U.S. Pat. No. 5,724,729, the stress exerted on a chip is not dispersed because there exists a gap between adjacent chips. Consequently, when a stress is applied to a certain chip, the bonded end face between the stressed chip and the heat spread cap is apt to be cracked. Moreover, since an underfill resin is not filled around the bump electrodes connecting the chips to the wiring board, the bump electrodes are apt to be oxidized, besides, heat generated in each chip is difficult to be transmitted to the wiring board.
In the multi-chip module disclosed in Japanese Patent Laid-open No. Sho 63-29563, plural semiconductor devices are fixed to the heat spread plate mechanically through the heat transfer part, so if there are variations in size of the semiconductor devices, the heat spread plate will tilt, making it difficult to fix the semiconductor devices to the heat spread plate. In the module disclosed in Japanese Patent Laid-open No. Sho 63-29563, since the heat transfer part is in the shape of comb teeth, it is difficult for the heat spread plate to tilt even if there are slight variations in size of the semiconductor devices, but the comb teeth shape makes the structure of the multi-chip module complicated. Although resin is molded sideways of each semiconductor device, there is a resin-to-resin gap, so that water is apt to get into the gap or moisture condensation is apt to occur, with a consequent tendency toward material deterioration of the resin and insulation degradation.
In a multi-chip module, it is well known, as described in Japanese Patent Laid-open No. Hei 7-86492, that an underfill resin is filled between the semiconductor chips and a wiring board. However, the provision of a heat spread plate is not described in Japanese Patent Laid-open No. Hei 7-86492, giving no hint for solving the foregoing problems.
It is an object of the present invention to provide a semiconductor module having a structure wherein a plurality of bare or packaged semiconductor chips are mounted on a single wiring board and are covered with a single heat spread plate, the semiconductor chips and the heat spread plate being bonded together so as to permit thermal conduction without using any other heat transfer part, with a stress exerted on each of the semiconductor chips being dispersed, as well as a mounting method for the semiconductor module.
It is another object of the present invention to provide a semiconductor module having a structure wherein a plurality of semiconductor chips are connected to a single wiring board with use of metal bumps, a single heat spread plate is disposed on the backs of the semiconductor chips, a gap between the semiconductor chips and the wiring board is filled with an underfill resin, and the semiconductor chips and the heat spread plate are bonded together without using any other heat transfer part, allowing a stress exerted on each chip to be dispersed, as well as a mounting method for the semiconductor module.
The present invention, in a first aspect thereof, resides in a semiconductor module having a structure wherein a plurality of bare or packaged semiconductor chips are mounted on a single wiring board and a single heat spread plate is disposed on the plural semiconductor chips, characterized in that resin is filled around the plural semiconductor chips thus sandwiched between the wiring board and the heat spread plate and that adjacent semiconductor chips are connected together through the resin.
As methods for mounting a bare or packaged semiconductor chip onto a wiring board there are a method wherein the semiconductor chip is mounted while a circuit-formed side of the chip faces down and is opposed to the wiring board, and a method wherein the circuit-formed side of the chip faces up and a circuit-free side of the chip is opposed to the wiring board. In a semiconductor module of a face-down bonding type wherein a semiconductor chip is connected to a wiring board while a circuit-formed side of the semiconductor chip faces down, pads formed on the circuit-formed side of the chip and pads formed on the wiring board are connected together through metal bumps. On the other hand, in a semiconductor module of a face-up bonding type wherein a semiconductor chip is mounted on a wiring board while a circuit-formed side of the chip faces up, a circuit-free side of the semiconductor chip is bonded to the wiring board using an adhesive, and pads formed on the circuit-formed side of the semiconductor.chip and pads formed on the wiring board are bonded together with metal wires. The term xe2x80x9cpadsxe2x80x9d as used herein indicate input-output terminals, which are sometimes called electrodes, electrode terminals, or simply terminals.
The present invention, in a second aspect thereof, resides in a semiconductor module having a structure wherein a plurality of either bare semiconductor chips or packaged semiconductor chips are connected to a single wiring board electrically through metal bumps while circuit-formed sides of the chips face down, and a single heat spread plate is disposed above circuit-free sides of the chips, characterized in that a resin is filled between the wiring board and the semiconductor chips, also between the semiconductor chips and the heat spread plate, and further between adjacent semiconductor chips, so that the semiconductor chips and the heat spread plate are bonded together with the resin, and the semiconductor chips are interconnected through the resin.
The present invention, in a third aspect thereof, resides in a semiconductor module having a structure wherein circuit-free sides of a plurality of bare or packaged semiconductor chips are bonded to a single wiring board, a single heat spread plate is disposed on circuit-formed sides of the semiconductor chips, and pads formed on the circuit-formed sides and pads formed on the wiring board are bonded together with metal wires, characterized in that resin is filled between the semiconductor chips and the heat spread plate and also between the plural semiconductor chips, so that the semiconductor chips and the heat spread plates are bonded together with the resin, and the semiconductor chips are interconnected through the resin.
In fabricating the semiconductor module according to the present invention, the semiconductor chips and the heat spread plate may be bonded together using a thermally conductive adhesive. In the semiconductor module of the type wherein semiconductor chips are metal wire-bonded to a wiring board while their circuit-formed sides face up, a spacer may be disposed between the heat spread plate and the semiconductor chips or between the heat spread plate and the wiring board to prevent metal wires from being crushed by the heat spread plate.
The present invention, in a further aspect thereof, resides in a semiconductor module having a structure wherein a plurality of bare or packaged semiconductor chips are mounted on a single wiring board and a single heat spread plate is disposed on upper surfaces of the plural semiconductor chips, characterized in that an adhesive layer is present between the semiconductor chips and the heat spread plate, that resin is filled around the semiconductor chips sandwiched between the wiring board and the heat spread plate, and that the semiconductor chips are interconnected through the resin.
The present invention, in a still further aspect thereof, resides in a semiconductor module having a structure wherein a plurality of bare or packaged semiconductor chips are mounted on a wiring board while circuit-formed sides thereof face up the semiconductor chips and the wiring board are metal wire-bonded to each other, and a single heat spread plate is disposed on the circuit-formed sides of the plural semiconductor chips, characterized in that a metal wire protecting spacer is disposed between the plural semiconductor chips and the heat spread plate, that resin is filled around the semiconductor chips sandwiched between the heat spread plate and the wiring board, and that the semiconductor chips are interconnected through the resin.
The present invention, in a still further aspect thereof, resides in a semiconductor module having a structure wherein a plurality of bare or packaged semiconductor chips are mounted on a wiring board while circuit-formed sides thereof face up, the semiconductor chips and the wiring board are metal wire-bonded to each other, and a single heat spread plate is disposed on the circuit-formed sides of the plural semiconductor chips, characterized in that a metal wire protecting spacer is disposed between the wiring board and the heat spread plate, that resin is filled around the semiconductor chips sandwiched between the heat spread plate and the wiring board, and that the semiconductor chips are interconnected through the resin.
The semiconductor module according to the present invention is obtained by mounting a plurality of semiconductor chips on a wiring board, disposing a heat spread plate, and pouring resin between the wiring board and the heat spread plate. As a resin pouring method there may be adopted a method wherein resin lumps or pellets are put respectively on upper surfaces of semiconductor chips a heat spread plate is then put thereon, then pressing plates are disposed outside both the wiring board and the heat spread plate and pressing is performed under heating, allowing the resin to melt or soften and flow and thereby allowing it to be filled around the semiconductor chips. There also may be adopted a method wherein a wiring board a heat spread plate and semiconductor chips are placed in a mold and a resin is poured into the mold by a transfer press molding. There also may be adopted a method wherein resin lumps or pellets are placed between semiconductor chips mounted on a wiring board and a heat spread plate, which are then placed together into an autoclave, followed by heating to melt or soften and flow the resin, allowing the resin to be filled around the semiconductor chips.
The present invention in still further aspects will become more apparent from working examples thereof to be described later.
The semiconductor chips used in the present invention are bare or packaged semiconductor chips. Bare chips are each formed with a circuit on one side thereof and are each provided with input-output terminals, designated pads, terminals, electrodes, or electrode terminals. In many cases, the input-output terminals are formed on a circuit-formed side of each semiconductor chip.
As an example of a packaged semiconductor chip there is mentioned CSP. An example of CSP is described in Japanese Patent Laid-open No. Hei 9-321084.
According to the CSP described therein, a wiring tape is disposed through a stress buffer layer on a circuit-formed side of a chip and is electrically connected to pads on the chip which connected portions are sealed using a resin, with metal bumps being formed on the wiring tape. The CSP of this structure is suitable for use in the present invention. Examples of such packaged semiconductor chips include SOJ (Small Outline J-lead package). TSOP (Thin Small Outline Package), and TCP (Tape Carrier Package), in addition to CSP, all of which are employable in the present invention.
In the present specification, the term xe2x80x9csemiconductor chipxe2x80x9d cover both bare chips and packaged semiconductor chips unless otherwise mentioned. Further, the term mountingxe2x80x9d as referred to herein refers, in a narrow sense, to a technique of mounting a semiconductor chip on a wiring board and connecting it to the board electrically, while in a broad sense, it refers to the technique plus a subsequent heat spread plate bonding technique or resin pouring technique. The xe2x80x9cmetal bumpxe2x80x9d as referred to herein corresponds to the bump electrode referred to in the prior art.
In the semiconductor module according to the present invention, since semiconductor chips are interconnected through a resin, even if a stress is exerted on any of the chips, it is dispersed in all directions through the resin. Thus, there is no fear of the chips and the heat spread plate being cracked. Besides, since the bonding between the semiconductor chips and the heat spread plate is produced by the resin, the heat spread plate is not likely to tilt, unlike the case where a heat transfer part is used. Further, in the semiconductor module having a structure wherein metal bumps are formed between semiconductor chips and a wiring board, since resin is filled between the chips and the wiring board, the thermal conductivity from the chips to the wiring board is improved and the metal bumps are difficult to be oxidized. The resin also serves to protect the chips and prevent bending of the module.
A semiconductor device having a structure in which a semiconductor chip is sandwiched between a substrate and a heat spread plate and a resin is molded around the chip, is disclosed in Japanese Patent Laid-open Nos. Hei 7-11278 and Hei 9-17827. But, the assembly of a multi-chip module is not described therein.
Since the space around each semiconductor chip sandwiched between the wiring board and the heat spread plate, which is filled with the resin, is narrow, it is desirable that the resin be superior not only in its fluidity but also in thermal conductivity because the resin also serves to let the heat of the semiconductor chip escape to the wiring board and the heat spread plate. More specifically, it is preferable for the resin to have a thermal conductivity in the range of 0.5 to 3.5 W/mxc2x0 C. If the thermal conductivity of the resin is less than 0.5 W/mxc2x0 C., a satisfactory heat dispersion effect will not be obtained, while if it exceeds 3.5 W/mxc2x0 C., then if the resin is of the type which requires the incorporation of a thermally conductive filler therein to impart a thermal conductivity to the resin, the amount of the filler becomes too large, with the result that the fluidity of the resin is deteriorated and thereby voids easily occur or an unsatisfactory fill results.
A thermosetting resin is one of the preferred resins. If a thermosetting resin is used, the resin viscosity will lower greatly at the time of heating for molding, so that the resin can be filled in a void-free state and the delamination of the resin becomes difficult to occur. As to a thermoplastic resin, a lowering of viscosity at the time of fill of the resin and the heat resistance of a resin composition obtained after cooling are not compatible with each other, so in the case of using a thermoplastic resin, it is desirable for the resin to be mixed with a thermosetting resin component. By incorporating a thermosetting resin into the thermoplastic resin, a three-dimensional network structure is introduced into the thermoplastic resin by heat-curing after molding and there is obtained a resin having a structure called an interpenetrating polymer network. The resin having such a network structure is improved in heat resistance while making the most of the advantage of the thermoplastic resin, namely, a short molding time.
As thermosetting resins there may be used, for example, acid anhydride curing type, phenol curing type, imidazole curing type, or amine curing type exposy resins. There also may be used polyfunctional acrylate and methacrylate resins, cyanate ester resins, and addition type maleimide or bismaleimide resins.
As thermoplastic resins there may be used polyimide or polyamide resins for example.
As to a thermosetting resin to be incorporated into the thermoplastic resin used, it is desirable to use a thermosetting resin having a linear expansion coefficient of 10 to 60 ppm/xc2x0 C., a glass transition temperature of not lower than 80xc2x0 C., and an elastic modulus at room temperature in the range of 500 MPa to 25 GPa, as characteristics of a heat-cured product obtained from the thermosetting resin. If the thermosetting resin is less than 10 ppm/xc2x0 C. in its linear expansion coefficient, lower than 80xc2x0 C. in its glass transition temperature, and less than 500 MPa in its elastic modulus at room temperature, it becomes easier for the resin to come off from the wiring board and the heat spread plate at the bonded portions due to a shrinking stress of the resin. If the linear expansion coefficient of the thermosetting resin exceeds 60 ppm/xc2x0 C. and the elastic modulus at room temperature thereof exceeds 25 GPa, there is an increase in the stress which is induced by a difference in thermal expansion coefficient between the semiconductor chip and the resin or between the wiring board and the resin, so that cracks and delamination are apt to occur in the interfaces of chips, metal bumps, and wiring board.
A thermoplastic polyimide resin with an epoxy resin, a maleimide resin or a bismaleimide resin as a thermosetting resin incorporated therein is well balanced in both adherence and hygroscopicity and is therefore suitable for use in the present invention.
From the standpoint of improving the adhesive properties and lowering the stress, it is desirable to incorporate a toughening agent into the resin used, such as silicone rubber, acryl rubber, or nitrile-butadiene rubber. Further, an inorganic filler may be incorporated in the resin for adjusting the linear expansion coefficient or for enhancing the thermal conductivity. As an inorganic filler it is desirable to use a spherical filler having a maximum particle size of not larger than 50 xcexcm and an average particle diameter in the range of 0.5 to xcexcm. If the maximum particle diameter of the inorganic filler used exceeds 50 xcexcm, the resin fill property into a narrow space is deteriorated and voids are apt to occur. If an average particle diameter of the inorganic filler used is smaller than 0.5 xcexcm, unfilling of the resin is apt to occur because the viscosity of the resin composition concerned increases abruptly. An extremely fine filler is apt to agglomerate and is inferior in dispersibility. Also, when the average particle diameter of the inorganic filler used exceeds 10 xcexcm, unfilling of the resin is apt to occur. For attaining a low thermal expansion coefficient of the resin it is desirable to use fused silica, synthetic silica, talc, or calcium carbonate as an inorganic filler. Among them, fused silica and synthetic silica are particularly preferred in point of purity, uniform particle shape, and low thermal expansibility. For enhancing the thermal conductivity of the resin it is desirable to use, for example, such a filler as crystalline silica, aluminum nitride, silicon nitride, boron nitride, or alumina. From the standpoint of decreasing the resin viscosity and reducing the damage to chips it is desirable that the above inorganic fillers be used in a spherical shape or a comer-cut-off shape close to a spherical shape.
In the case where the gap between the wiring board and the heat spread plate is to be filled with a thermosetting resin alone, it is desirable to use a thermosetting resin having a linear expansion coefficient of not more than 60 ppm/xc2x0 C., especially not more than 40 ppm/xc2x0 C.
As the material for the heat spread plate, there may be used, for example, any of such metals as copper, aluminum, stainless steel, iron, and kovar, as well as such ceramic materials as aluminum nitride, silicon nitride, boron nitride, and alumina, may be used. There also may be used a resin plate containing a filler superior in thermal conductivity such as aluminum nitride, silicon nitride, boron nitride, or alumina, or a resin plate containing a metal as core. A copper plate plated with nickel is also available.
From the standpoint of diminishing warp of the semiconductor module, it is desirable for the heat spread plate, to have a linear expansion coefficient close to that of the wiring board. It is preferable that the difference in linear expansion coefficient between the wiring board and the heat spread plate be not larger than 10 ppm/xc2x0 C., more preferably 5 ppm/xc2x0 C. If the difference in linear expansion coefficient between the two exceeds 10 ppm/xc2x0 C., the module is apt to be warped and the resin is cracked easily in a temperature cycle test. In a temperature cycle condition of xe2x88x9255xc2x0 C. to 150xc2x0 C., it is impossible to satisfy 1,000 cycles unless the difference in linear expansion coefficient between the two is 5 ppm/xc2x0 C. or less.
It is preferable for the heat spread plate used to have a thickness in the range of 10 xcexcm to 2 mm. A plate thickness thereof smaller than 10 xcexcm is too thin and makes its handling difficult. If the plate thickness exceeds 2 mm, it is difficult to cut the plate and burrs are apt to occur; besides, the thickness of the semiconductor module itself becomes larger, so such a plate thickness is not suitable for the formation of a thin module. The heat spread plate may be in the shape of a single flat plate or in a cap shape obtained by bending end portions of a plate.
Adhesion of the heat spread plate and the semiconductor chips is used mainly with resin and the heat of each chip is transferred to the heat spread plate through the resin. Therefore, it is preferable that the spacing between the chips and the heat spread plate be as narrow as possible. However, if the spacing is too narrow, voids and delamination of the resin are apt to occur. In view of this point, it is desirable that the spacing of the two be in the range of 10 to 200 xcexcm.
As the material for the wiring board, there may be used, for example, an organic printed wiring board containing a woven or non-woven fabric formed by glass fibers or organic fibers, a wiring tape such as a polyimide tape, or a ceramic substrate, according to the purpose for which the wiring board is to be used. An organic printed wiring board is suitable for a semiconductor module of a low cost. A flexible polyimide wiring tape is suitable for a semiconductor module which requires the formation of a fine pitch. Further, a ceramic substrate is suitable for a semiconductor module for which high heat resistance and high reliability are required such as a semiconductor module used in an automobile or an industrial electronic device.